Sensory experience powerfully regulates late postnatal development and adult function of brain circuits, particularly in the cerebral cortex. The cellular mechanisms that mediate this process are not yet understood, but have major implications for understanding cortical neurodevelopmental disorders, including autism, juvenile epilepsy, and mental retardation. This project focuses on novel mechanisms by which experience controls the balance of excitation and inhibition in cerebral cortex. This is studied in the somatosensory cortex of rodents, which is a canonical model system for cortical function. Standard models of experience-dependent development and modification (plasticity) focus on excitatory cortical circuits. However, recent findings indicate that inhibitory circuits also show robust plasticity by sensory experience. The prevalence, cellular mechanisms, and mechanistic role of inhibitory plasticity are largely unknown. Using cellular and systems-level neurophysiology techniques, we will identify specific inhibitory neurons and circuits that are regulated by sensory experience, characterize the cellular mechanisms for this plasticity, and determine its role in cortical function. We specifically test the hypothesis that inhibitory plasticity has a dual role: to maintain excitatory-inhibitory balance, and to mediate rapid homeostasis of sensory responses during changing sensory use. Substantial preliminary data support the proposal. Overall, this work will extend our understanding of cortical development beyond basic plasticity of excitatory circuits, to include rapid, robust plasticity of inhibition. Results may suggest a novel basis for neurodevelopmental disorders including juvenile epilepsy and autism, which may arise from improper development of excitatory-inhibitory balance. This work may lead to improved animal models and novel therapeutic strategies for these diseases.